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Key Points

  • Disease summary:

    • Myotonic dystrophy type 1 is a multisystemic disease with variable clinical manifestations.

    • Muscle weakness typically involves the face, long finger flexors, intrinsic hand muscles, and foot dorsiflexors.

    • Myotonia (impaired muscle relaxation) may be elicited with grip or percussion of the muscle. Myotonia is not present in infants or very young children with other features of myotonic dystrophy.

    • Early-onset cataract (age <50) is often present and may be the only disease feature in mildly affected individuals.

    • Severe cases of myotonic dystrophy type 1 may result in onset of symptoms at birth, known as congenital myotonic dystrophy. Features associated include hypotonia, respiratory failure, feeding difficulties, clubfoot, and cognitive delay.

    • Myotonic dystrophy type 1 affects the heart, smooth muscle, gastrointestinal (GI) system, endocrine system, central nervous system (CNS) in addition to the core features, as delineated in Table 131-1.

  • Hereditary basis:

    • Myotonic dystrophy type 1 is an autosomal dominant condition due to an unstable CTG repeat expansion in the 3’untranslated region of the DMPK gene on chromosome 19q13.3. It has complete penetrance but disease manifestations are subject to anticipation. As such, subsequent generations have earlier onset of more severe symptoms due to greater expansion of the trinucleotide repeat length.

  • Differential diagnosis:

    • Myotonic dystrophy type 2 may present similarly in adults, though the weakness is typically more proximal. Isolated myotonia without weakness can also occur in nondystrophic myotonias (myotonia congentia). Congenital myopathies, spinal muscular atrophy, and Pompe disease are all considerations in the evaluation of a congenital myotonic dystrophy patient.

Table 131-1System Involvement

Diagnostic Criteria and Clinical Characteristics

Diagnostic Criteria

Myotonic dystrophy type 1 has the most varied presentation of any muscular dystrophy. However, certain features are highly associated with myotonic dystrophy type 1:

  • Early-onset cataracts (age <50)

  • Muscle weakness, especially long finger flexors


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